The present invention relates to microelectromechanical devices and associated fabrication methods and, more particularly, to microelectromechanical devices having both single crystalline components and metallic components as well as the associated fabrication methods.
Microelectromechanical structures (MEMS) and other microengineered devices are presently being developed for a wide variety of applications in view of the size, cost and reliability advantages provided by these devices. Many different varieties of MEMS devices have been created, including microgears, micromotors, and other micromachined devices that are capable of motion or applying force. These MEMS devices can be employed in a variety of applications including hydraulic applications in which MEMS pumps or valves are utilized, optical applications which include MEMS light valves and shutters, and electrical applications which include MEMS relays.
MEMS devices have relied upon various techniques to provide the force necessary to cause the desired motion within these microstructures. For example, electrostatic actuators have been used to actuate MEMS devices. See, for example, U.S. patent application Ser. No. 09/320,891, assigned to MCNC, also the assignee of the present invention, which describes MEMS devices having electrostatic microactuators, the contents of which are incorporated herein by reference. In addition, controlled thermal expansion of an actuator or other MEMS component is another example of a technique for providing the necessary force to cause the desired motion within MEMS structures. See, for example, U.S. Pat. No. 5,909,078 and U.S. patent application Ser. Nos. 08/936,598; and 08/965,277, assigned to MCNC, also the assignee of the present invention, which describe MEMS devices having thermally actuated microactuators, the contents of which are incorporated herein by reference.
An example of a thermally actuated microactuator for a MEMS device comprises one or more arched beams extending between a pair of spaced apart supports. Thermal actuation of the microactuator causes further arching of the arched beams which results in useable mechanical force and displacement. The arched beams are generally formed from nickel using a high aspect ratio lithography technique which produces arched beams with aspect ratios up to 5:1. Although formed with high aspect ratio lithography, the actual nickel arched beams have rather modest aspect ratios and may therefore have less out-of-plane stiffness and be less robust than desired in some instances. Further, the lithography technique used to form nickel arched beams may result in the arched beams being spaced fairly far apart, thereby increasing the power required to heat the arched beams by limiting the amount that adjacent arched beams heat one another. In addition, the resulting microactuator may have a larger footprint than desired as a result of the spacing of the arched beams. Thus, there exists a need for arched beams having higher aspect ratios in order to increase the out-of-plane stiffness and the robustness of microactuators for MEMS devices. In addition, there is a desire for microactuators having more closely spaced arched beams to enable more efficient heating and a reduced size.
Nickel microactuators are typically heated indirectly, such as via a polysilicon heater disposed adjacent and underneath the actuator, since direct heating of the nickel structure (such as by passing a current therethrough) is inefficient due to the low resistivity of nickel. However, indirect heating of the microactuator of a MEMS device results in inefficiencies since not all heat is transferred to the microactuator due to the necessary spacing between the microactuator and the heater which causes some of the heat generated by the heater to be lost to the surroundings.
Nickel does have a relatively large coefficient of thermal expansion that facilitates expansion of the arched beams. However, significant energy must still be supplied to generate the heat necessary to cause the desired arching of the nickel arched beams due to the density thereof. As such, although MEMS devices having microactuators with nickel arched beams provide a significant advance over prior actuation techniques, it would still be desirable to develop MEMS devices having microactuators that could be thermally actuated in a more efficient manner in order to limit the requisite input power requirements.
The above and other needs are met by the present invention which, in a preferred embodiment, provides a microelectromechanical device comprising a microelectronic substrate, a microactuator disposed thereon and comprised of a single crystalline material, such as silicon, and at least one metallic structure disposed on the substrate in a spaced relationship from the microactuator and preferably in the same plane as the microactuator such that the microactuator can contact the metallic structure upon thermal actuation thereof. In particular, actuation of the microactuator causes said at least one metallic structure to be engaged and moved as a result of the operable contact with the microactuator. In one advantageous embodiment, the MEMS device may include two adjacent metal structures with one of the metallic structures being fixed and the other metallic structure being moveable. In this embodiment, the MEMS device may be a microrelay such that actuation of the microactuator brings the microactuator into operable contact with the moveable metallic structure, thereby permitting the metallic structures to be selectively brought into contact in response to actuation of the microactuator.
According to one advantageous embodiment, the microactuator is thermally actuated. In this embodiment, the microactuator preferably comprises a pair of spaced apart supports disposed on the substrate and at least one arched beam extending therebetween. The microactuator may also include an actuator member that is operably coupled to the at least one arched beam and extends outwardly therefrom. The microactuator further includes means for heating said at least one arched beam to cause further arching thereof, wherein the actuator member moves between a first position in which the actuator member is spaced apart from said at least one metallic structure and a second position in which the actuator member operably engages said at least one metallic structure.
In another embodiment of the present invention, the microactuator is electrostatically actuated. In this embodiment, an electrostatic microactuator may comprise, for instance, a microelectronic substrate having at least one stator disposed thereon. Preferably, the stator has a plurality of fingers protruding laterally therefrom. Further, the electrostatic microactuator includes at least one shuttle disposed adjacent the stator, wherein the shuttle is movable with respect to the substrate and has a plurality of fingers protruding laterally therefrom. The fingers protruding from the shuttle are preferably interdigitated with the fingers protruding from the stator. An actuator member is coupled to the shuttle, protrudes outwardly therefrom, and extends between a pair of spaced apart supports. Electrical biasing of the stator with respect to the shuttle causes movement of the shuttle such that the actuator member operably engages the metallic structure in response to the actuation of the electrostatic actuator.
Another advantageous aspect of the present invention comprises the associated method to form a microelectromechanical device having both single crystal components and metallic components. According to one preferred method, a microactuator, such as a thermally actuated microactuator or an electrostatic microactuator, is formed from a wafer comprised of a single crystalline material. At least one metallic structure is also formed upon a surface of a substrate such that at least one metallic structure is moveable relative to the substrate. The microactuator is then bonded upon the surface of the substrate such that portions of the microactuator are also moveable relative to the substrate in order that the microactuator may operably engage the metallic structure in response to thermal actuation thereof.
An alternative method of fabricating a microelectromechanical device having both single crystal components and metallic components in accordance with a preferred embodiment of the present invention comprises bonding a wafer comprised of a single crystalline material upon a surface of a substrate. After polishing the wafer to the desired configuration, at least one window may be defined through the wafer, extending to the substrate. Using the wafer as a template, at least one metallic structure may then be formed within said at least one window defined by the wafer and upon the surface of the substrate. A portion of the wafer surrounding the at least one metallic structure can then be etched away to permit the metallic structure to be moveable relative to the substrate. Either before or after the metallic structure is formed, a microactuator is formed from the wafer such that portions of the microactuator are moveable relative to the substrate and are capable of operably engaging the metallic structure in response to thermal actuation thereof.
Yet another alternative method of fabricating a microelectromechanical device having both single crystal components and metallic components in accordance with a preferred embodiment of the present invention comprises bonding a wafer comprised of a single crystalline material upon a surface of a substrate. After polishing the wafer to the desired configuration, a portion of the wafer can be etched away and at least one metallic structure formed upon the surface of the substrate such that the metallic structure is moveable relative to the substrate. Either before or after the metallic structure is formed, a microactuator is formed from the wafer such that portions of the microactuator are moveable relative to the substrate and are capable of operably engaging the metallic structure in response to thermal actuation thereof.
Thus, a MEMS device, such as a microrelay, can be formed in accordance with the present invention that includes actuators formed of single crystalline silicon, while other components of the MEMS device are formed of metal, such as nickel. Fabricating, for example, the arched beams of a thermally actuated microactuator or the interdigitated fingers of an electrostatic microactuator from single crystalline silicon allows the features to be formed with aspect ratios of up to at least 10:1, particularly by using a deep reactive ion etching process. The higher aspect ratios of the features and components increases their out-of-plane stiffness and constructs a more robust device. The fabrication techniques of the present invention also advantageously permit closer spacing of features and components. For example, the closer spacing between adjacent silicon arched beams of a thermally actuated microactuator results in more effective transfer of heat between adjacent arched beams. In addition, the single crystalline silicon microactuator can be directly heated, such as by passing a current therethrough. As will be apparent, direct heating of the microactuator is generally more efficient than indirect heating. Further, although the coefficient of thermal expansion of silicon is less than that of metals, such as nickel, silicon is significantly less dense than nickel such that for a given amount of power a silicon arched beam can generally be heated more than a corresponding nickel arched beam. Therefore, the MEMS device of the present invention can have greater out-of-plane stiffness, can be more robust and can be more efficiently heated than conventional MEMS microactuators having metallic components.